DH

I gained a deeper understanding and greater appreciation for how my body works. Thank you to the Teams at Duke and Coursera for making such a wide body of information available to the motivated.

WL

Apr 01, 2016

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Lectures are very concise. This course can help you grasp the most important core concepts and key woking mechanisms of human physiology. But I don't know why it do not have the immune system.

수업에서

Homeostasis and Endocrine System

Welcome to Module 2 of Introductory Human Physiology! We begin our study of the human body with an overview of the basic concepts that underlie the functions of cells and organs within the body and their integration to maintain life. This is an important introduction to how physiologists view the body. We will return to these basic concepts again as we progress through the organs systems and consider how they respond to perturbations incurred in daily functions and in disease.
The things to do this week are to watch the 6 videos, to answer the in-video questions, to read the notes for each topic, and to complete two problem sets (homeostasis, transporters & channels, and endocrine concepts). It will be most effective if you follow the sequence of videos. The notes provide a more detailed summary of each topic. We encourage you to find which resource (videos and/or notes) works best for you.We have included a set of problems to be completed as homework exercises. We strongly encourage you to complete these problems sets. They are not graded and are for your personal feedback. It has been our experience that these exercises are helpful in increasing understanding and retention of the newly learned materials.Please use the interactive forum as a means to exchange ideas, to ask questions, to form study groups and interest groups, and to meet your community. We will monitor the forum daily.Thank you for joining us. We are excited about sharing this educational experience with you. Welcome!

강사:

Jennifer Carbrey

Assistant Research Professor

Emma Jakoi

Associate Research Professor

스크립트

Greetings so today we want to talk about how we move water across the plasma membrane. And remember the last time we said that the plasma membrane was a bilayer of lipids. So that it was a hydrophobic barrier around all the cells of the body. In order to move a molecule that's hydrophilic across this barrier, we had to have some type of protein which was a transporter or a pump. In the case of water, water is a hydrophilic molecule, it's a polar molecule and so water by itself is very very slow to move across the plasma membrane. But water has its own very specific transporter and this specific transporter is called aquaporin. The aquaporin is present in essentially all cells of the body, so water is able to move across the plasma membrane very rapidly through this channel because the channel is open at all times. It is not a gated channel. So the movement of water because it's so important to understanding how we control volume of cells has this very unspecial name and that's called osmosis. And so today we're going to talk about osmosis and secondly we want to consider the terms osmoliarity and tonicity which are terms which will govern the movement of water. And then lastly, we want to talk about how these effective solutes, these are solutes which are not permeable to the plasma membrane are good to regulate the size of the fluid compartments of the body. All right, so the first thing we have to think about, and this is not always intuitive, and that is the concentration of water. Because water is going to be moving by facilitated diffusion through that aquaporon channel. The aquaporin channel is open at all times, so it is not a gated channel, the concentration of water is highest in pure water. And when we add a solute to water, we will then decrease the concentration of the water. So, for instance, if this particular vessel is one liter in volume and the vessel over here is one liter in volume by adding the sodium to the second vessel. This is to this one. And what happens is that the concentration of the water is less in vessel two than it is in vessel one. And I know you're all sitting there saying, well that's pretty obvious. But sometimes its confusing so let's just think about that and try to remember that water, the highest concentration of water is pure water. All right, so let's look at why this can be very important. If we have 2 different compartments, so we have two cells, we have cell 1 and we have cell 2. In cell one, we have two sodium ions and in cell two we have four. Now these have equal compartments, so they're the same size, so let's say this is one liter in size and this is one liter in size. If we allow these two cells to come together and we have a membrane between them that allows the movement both of the solute and of the water. Then the solute and the water will reach equilibrium, so the sodium will diffuse from one cell to the other. So will have been an equal number sodiums' in cell one as we have in cell two and water will also distribute equally between the two so that the concentration of the sodium will be equal in both cases. And that's pretty straightforward. Now notice that we did not change the volume of the two compartments, so they're each one litre in size. But what happens if we take our same two cells and we put them together, but now we put a membrane between those cells which is not permeable to the solute. It is permeable to water, so this sodium all stay in compartment 2 and the water now it's in compartment one, is able to leave compartment 1 and enter into compartment 2. To dilute compartment 2, so that the concentration in 2 is equal to the concentration in 1 and that's what shown here. And notice that by doing so we now have change the volume of compartment two. So were this use to be compartment one used to be one liter. Now let's say its 500 milliliters, it's half and compartment 2 is now increased by another half. So it's now 1.5 L, so the diffusion of water then, is a facilitated diffusion. Diffusion of water requires the aquaporin channels, and it will cause the change in the compartment size when the membrane is impermeable to the solute. And this is a key thing to remember because if we were talking about cells in your body, the cells in your body are going to respond in exactly the same way. So that if we have a concentration change in the ECF, such that you eat a lot of sodium, so that the ECF now has a lot of sodium in it. The water that's within the cells will leave the cells and move to the ECF so that the concentration of the sodium will be equalized. A concentrations between ECF and the ICF will be equal, and we'll talk about this in just a second, so osmosis then, is the movement of water. It occurs by diffusion only, so we're going from a high concentration of water, to a low concentration of water. We use aquaporin channels, and this is facilitated diffusion so that moves very quickly when we have these aquaporin channels present. And the channels are not gated, the channels always open so we a patent opening between the cells and the highest concentration of water is pure water. So we want to talk about two separate concepts. One is the osmolarity of a solution, and the second is the tonacity of the solution. So when we calculate osmolarity of the solution, we need to calculate how many particles are within the solution. Not just the number of moles that are within the solution. Normally, when you think of a solution, you think of the molarity of the solution and that's the number of moles/vol and that's what's shown here. But the osmolarity then we also consider the number of particles. Okay, so let's think about this, so we have a solution where we have a one Molar solution of sodium chloride,. And on molar solution of sodium chloride, the sodium and the chloride dissociate into two particles. So those two particles then means that we will have two osmolar solution of sodium chloride. There's also a term called osmolality, and in biological systems, we really don't make a very large distinction between osmolality and osmolarity. The difference is that in osmolarity, we're talking about one liter for our volume and osmoality. We're talking about a kilogram of water for our volume but we will consider in this course, we'll consider them to be essentially equivalent. The other thing we're going to consider is that in body, the osmolarity of the cell is about 300 mOsM. So the body is going to be 300 milliOsmols and if I put this cell into a solution of ECF and the ECF is 300 mOsM, then that solution is iso-osmotic. To the cell because it's the same osmolarity as the cell. If I have the ECF is actually 200 milliosmolars, then it is more dilute than the cell and so then it would be called hypoosmotic to the cell. And if the solution that I put the cell into is 400, then 400 miOsM, then that's a hyperosmotic solution relative to the cell. Okay, iso meaning the same or equal. Hypo meaning less and hyper meaning more. Now, when we're calculating osmolarity, we calculate all the molecules that are within the solution. So, if I have one molar sodium chloride and I add to that a molar of urea And urea does not dissociate into more than one particle, then that solution becomes a 3 OsM solution. Sodium Chloride plus urea makes a 3 OsM solution. Now the difference between osmolarity and tonicity is that with tonicity, we do not count all of the particles that are in the solution. We only count the particles that are non penetrating and the non penetrating particles means that they cannot go across the plasma membrane. Remember, urea could go across the plasma membrane but a non-penetrating particles would be the sodium and the chloride. If I had my red blood cell and I put it into a solution that's 300 mOsM, Then the red blood cell is happy because it itself is 300 mOsM and that's going to be a solution that's isotonic. Is the seeing tonicity as the cell. If I then dilute the solution that the cell is sitting in, then the solution could go down to let's say 200 mOsM. And then what that happens the red blood cell which is at 300 will take in water because the solution now is hypotonic to the red blood cell. And the water is going to move from a higher concentration, which is outside of the cell, across the membrane, and into the red blood cell and our red blood cell swells. Conversely, if I put our red blood cell, where the solution is now 400 mOsM. Now, this solution if hypertonic to the cell. The cell will shrink and the water will then move out of the cell and into it's environment. So the cell then, is going to try to balance, the water is going to try to balance the concentration of the solution that's around the cell. And it does so by moving across that aquaporon channel. So the tonicity then, we have to consider the non-penetrating molecules only. So in a solution where we have a 1 milliosmole solution of sodium chloride and we add to it a 1 milliosmole solution of urea. That solution would still be on a 1 milliosmolar because we don't consider the urea. The urea can go across the plasma membrane. All right, so why am I torturing you with this? This is a really important point. Several years ago there was some runner who in the Boston marathon who had over hydrated and they had over hydrated as they were running their race. And what happened is that they diluted down their ECF, they diluted down the actual osmolarity of their blood. And by diluting down the osmolarity of their blood then they have a situation where water started moving into the neurons of the brain and the neurons of the brain started to sell. Three of these runners actually dies from this .so this is a really important point that we need to adjust the amount of tonicity, that is the amount of solutes that's within the ECF. And therefore, within the vasculature such that it's compatible with life. And that when we have very high salt within the ECF, water will move from the cells into the ECF. But if we had a very dilute solution in the ECF, then water will go the opposite direction in the cells as well. So one of things that the bodies going to want to do is to always maintain the ECF at about 300 millosmoles. So let's go through this table so that you can sort of think about what I'm talking about. So the first one is that we are going to give an IV, that is, by needle directly into a vein of an individual. And we're going to give them isotonic saline, so this is going to be 300 milliosmoles. So it's 300 milliosmolar solution. So the total body water of course will increase and the effect on the ECF osmolarity is that there's no change. But the ECF volume is going to increase because we're fusing, we're actually putting some solution into the body. Does the volume of the ICF changed? Does the volume of the cell change? And the answer is no, there is no change in the volume of the cells. Because the solution was isotonic to the cells, there is no loss of water or gain of water by the cells. On the other hand if we have a situation where we had diarrhea where we have an isotonic loss. So that we're losing an isotonic fluid from the body, from the anus, the total body water is going to decrease. The effect on the osmolarity, again, there's no change in the osmolarity because we're losing an isotonic solution from the body. The ECF volume decreases, but again there's no change in ICF volume. Now in the third case we take in excess amounts of sodium. You eat a really big bag of potato chips, salty, salty potato chips and you don't drink any water. And as you're taking in all that salt, the sodium then is coming into the body, so the sodium is coming into the body. So the ECF, the osmolarity of the ECF, increases because all of that sodium is going to go into the ECF. The volume of the body is not changing, I didn't not bring in any fluids. So the body volume, the total body water is staying the same, so under these conditions what happens to the ECF volume? I've brought a lot of sodium into the ECF volume. So now, the volume in the ECF is going to increase and the water is going to come from the cells. And so the water moves from the cells into the ECF and the cells will shrink. Right, so you think about this and you think about what happen in the last case. Where we had excess sweating.So there is a hypotonic loss from the body and figure out how the ECF volume and ICF volumes are going to be affected. All right, so what's our general concepts? So the first is we have two fluid compartments of the body. We have the intracellular fluid and we have the extracellular fluid and these are in osmotic balance. The second one is that the water moves by facilitated diffusion through aquaporin channels across most cell membranes and this process is called osmosis. Third, we have a non-permeable solutes are called effective solutes and these will affect the cellular volume. The cellular volume is critically dependent on the steady state of the effective solutes and the water across the cell membrane. If I increase the number of effective molecules in the ECF. Water will move from the cells to the ECF to try to balance the concentration across the two compartments. The last is that the cells will shrink in hypertonic ECF conditions and the cells will swell in a hypotonic ECF condition. All right, so the next time we meet, then, we're going to talk about one more of these general concepts, things that we're going to see as we go through the rest of the course. And as we go into the gastrointestinal tract and into the renal system, you may want to come back and look over this lecture and the lecture on the transporters. So see you next time.